Many circuits selectively receive inputs from and/or provide outputs to two or more other circuits. A switching circuit that includes transistors may be used to select between the inputs and/or outputs. For example in FIGS. 1A and 1B, first and second circuits 10 and 12 are selectively connected by a switching circuit 14 to a third circuit 16. In some implementations, the first and second circuits 10 and 12 are selectively connected by transistors Q1 and Q2 and Q3 and Q4, respectively. Switching inputs S1 and S1 are used to select the first circuit 10 or the second circuit 12. When S1 is in a first state, the first circuit 10 is connected and the second circuit 12 is not connected. When S1 is in a second state, the second circuit 12 is connected and the first circuit 10 is not connected.
In some situations, the output signal of the first and second circuits 10 and/or 12 may exceed the voltage supply and/or breakdown voltage of the transistors Q1, Q2, Q3 and Q4 that are used in the switching circuit 14. For example, a voltage supply that supplies the switching circuit 14 may provide 2.5V. The switching circuit 14 may be used to switch between first and second transmitters in an Ethernet network device. The voltage output of an exemplary transmitter in a 100BASET network may be operated with a maximum voltage of 3.5V, a minimum voltage of 1.5V, and a common mode voltage of 2.5V. The maximum voltage level of the transmitter outputs may cause operational problems such as breakdown of the transistors Q1, Q2, Q3, and Q4.
Another situation that may require analog switching includes switching between MDI and MDIX configurations in 100BASET or 10BASET network devices. Referring now to FIGS. 2A and 2B, first and second network devices 20 and 22 include physical layers (PHYs) 24 and 26, respectively, that are connected by network cables. For example, the network device 20 can be a personal computer or printer and the network device 22 can be a network switch. Each of the network devices 20 and 22 is connected by at least two pairs of twisted pair wires that are labeled 1, 2 and 3, 6 in FIGS. 2A and 2B.
When in an MDI configuration in FIG. 2A, the PHY 24 has a first pair 1, 2 that is configured as a transmitter 30 and a second pair 3, 6 that is configured as a receiver 34. When in an MDIX configuration in FIG. 2B, the PHY 24 has first pair 1, 2 that is configured as a receiver 46 and a second pair 3, 6 that is configured as a transmitter 48. When in an MDIX configuration, the PHY 26 has a first pair 1, 2 that is configured as a receiver 40 and a second pair 3, 6 that is configured as a transmitter 44. When the network devices 20 and 22 have different configurations, a standard or straight network cable 50 is used. When the network devices 20 and 22 have the same configuration, a crossover network cable 52 is used. When the incorrect network cable is employed for a particular situation (as in FIG. 2B), either the cable must be changed or the transmitter and receiver connections for one of the network devices needs to be switched.
Referring now to FIG. 12, a functional block diagram of a laptop docking system according to the prior art is presented. A laptop 402 is removably connected to a docking station 404. The laptop 402 includes a motherboard 406 and a first network connector 408. A physical layer (PHY) device 410 communicates with a switch 412 and a media access control (MAC) device 416, which are all arranged on the motherboard 406.
The PHY device 410 provides an interface to a physical medium such as coaxial cable, fiber optic cable, or twisted pair. The MAC device 416 provides an interface between the PHY device 410 and a host, such as a processor of the laptop 402. The docking station 404 includes a second network connector 414. The first and second network connectors 408 and 414 communicate with the switch 412.
The first and second network connectors 408 and 414 may include RJ-45 connectors. The switch 412 selectively connects the first network connector 408 or the second network connector 414 to the PHY device 410. When the laptop 402 is connected to the docking station 404, the switch 412 may automatically select the second network connector 414. Once the laptop 402 is removed from the docking station 404, the switch 412 may automatically select the first network connector 408.
The switch 412, however, has an inherent resistance. The resistance causes a voltage drop between the PHY device 410 and the first and second network connectors 408 and 414, which degrades performance. Incoming signals are attenuated, leading to a greater error rate in identifying received symbols. If the incoming signals are already attenuated, such as by a long twisted pair transmission line, the additional attenuation caused by the switch may cause the incoming signals to violate a minimum voltage specification.
The switch 412 causes similar attenuation problems for transmit signals. In order to decrease the resistance of the switch 412, the size of the switch 412 can be increased, as shown by a relationship illustrated in FIG. 13A. However, as the size of the switch 412 increases, the capacitance of the switch 412 also increases, as shown by a relationship illustrated in FIG. 13B.
As capacitance increases, the bandwidth of the switch 412 is limited, as shown in FIG. 13C. When the switch 412 is small enough to maintain adequate bandwidth for a protocol such as Gigabit Ethernet, it may have a resistance of approximately 5 ohms. With a termination resistance of 50 ohms, such as is typical of Ethernet, the resistance of the switch 412 causes an approximate 10% decrease in signal strength.
Referring now to FIG. 14A, a functional block diagram of a network interface according to the prior art with a single network connector is presented. A transmission line 500 communicates with the first network connector 408. The first network connector 408 communicates with a transformer 504, which couples signals from the transmission line 500 to a termination resistance 506. The termination resistance 506 communicates with a transmitter 508 and a receiver 510. A control module 512 transmits data to the transmitter 508 and receives data from the receiver 510. The control module 512 communicates with the MAC device 416.
Referring now to FIG. 14B, a functional schematic diagram of the network interface of FIG. 14A is presented. The transmission line 500 is coupled to the first network connector 408, which communicates with the transformer 504. The transformer 504 communicates with the termination resistance 506. The transmitter 508 provides a current Itx 520 to first and second ends of the termination resistance 506. The receiver 510 detects a voltage Vtx 522 across the first and second ends of the termination resistance 506.
Referring now to FIG. 15A, a functional block diagram of a switched network interface according to the prior art including two network connectors is presented. The docking station 404 includes the second network connector 414. For purposes of illustration, the transmission line 500 is shown coupled to the first network connector 408. The switch 412 selectively couples one of the first and second network connectors 408 and 414 to the transformer 504.
Referring now to FIG. 15B, a functional schematic diagram of the switched network interface of FIG. 15A is presented. For purposes of illustration, the transmission line 500 is coupled to the first network connector 408. The switch 412 selectively couples one of the first and second network connectors 408 and 414 to the transformer 504. The voltage Vtx 522 measured across the termination resistance 506 is reduced by the voltage drop in the switch 412.
Referring now to FIG. 16A, a functional block diagram of another switched network interface according to the prior art is presented. For purposes of illustration, the transmission line 500 is connected to the first network connector 408, which couples the transmission line 500 to the termination resistance 506 via the transformer 504. The termination resistance 506 communicates with the transmitter 508 and the receiver 510.
The termination resistance 506 may communicate with the transmitter 508 and the receiver 510 via a hybrid (not shown). The transmitter 508 and the receiver 510 communicate with the control module 512. The second network connector 414 communicates with a second transformer 540. The second transformer 540 communicates with a second termination resistance 542.
The second termination resistance 542 communicates with a second transmitter 544 and a second receiver 546. The second termination resistance 542 may communicate with the second transmitter 544 and the second receiver 546 via a hybrid (not shown). The second transmitter 544 and the second receiver 546 communicate with a second control module 548.
A switch 550 selectively connects the control module 512 and the second control module 548 to the MAC device 416. The docking station 404 includes the second network connector 414 and may also include the second transformer 540, the second termination resistance 542, the second transmitter 544, the second receiver 546, and the second control module 548. The expense of duplicating all these components makes this solution unattractive.
Referring now to FIG. 16B, a functional schematic diagram of the switched network interface of FIG. 16A is presented. For purposes of illustration, the transmission line 500 is coupled to the first network connector 408. The first network connector 408 communicates with the transformer 504, which in turn communicates with the termination resistance 506. The termination resistance 506 communicates with a transceiver module 560.
The transceiver module 560 includes a transmitter and a receiver, such as the transmitter 508 and the receiver 510 of FIG. 16A, and indicated by the current Itx 520 and the voltage Vtx 522, respectively. The second network connector 414 communicates with the second transformer 540, which in turn communicates with the second termination resistance 542. The second termination resistance 542 communicates with a second transceiver module 562. The switch 550 selectively connects the first and second transceiver modules 560 and 562 to the MAC device 416 of FIG. 16A.